Toilet Cistern Hissing or Running Non-stop? Causes and Fixes

Hearing water running in the bathroom when nobody flushed? Or the cistern refilling by itself every so often? Water goes straight to the drain, so nobody notices until the bill arrives. Most causes are simple and quick to fix.

How to tell if the cistern leaks

The food-colouring test

1. Lift the cistern lid.
2. Add a few drops of food colouring.
3. Don't flush; wait 15 minutes.
4. If colour reaches the bowl, the cistern is leaking.

Other signs

• Refilling sounds without use.
• A thin trickle running down the inside of the bowl.
• The cistern takes a long time to fill.
• You need to press the flush more than once.

Common causes

• Worn flush seal: the rubber seal at the bottom hardens or cracks with scale — the most common cause.
• Float set too high: water reaches the overflow and runs into the bowl constantly.
• Tired fill valve: scale or a worn washer stops it closing fully.
• Sticking flush button: on dual-flush cisterns the button or linkage can hold the valve slightly open.

What you can do yourself

• Close the cistern's isolation valve if the noise won't stop.
• Lower the float if water is reaching the overflow.
• Wipe scale off the seal — sometimes enough as a temporary fix.

Avoid strong chemicals inside the cistern; they damage the rubber parts.

When to call a plumber

Old internal parts, a concealed in-wall cistern you can't reach, a leak at the base of the toilet, or a rocking toilet all need a technician. We handle these under leak detection and bathroom work.

Summary

If you hear the cistern refilling on its own, do the colour test today. It's usually a small part — but leaving it means water wasted every hour. Contact us.